1. Field of the Invention
The present invention relates to toner compositions comprising modified pigments. The present invention further relates to processes for preparing toners and the toners resulting from these processes.
2. Description of the Related Art
Electrophotographic processes and image-forming apparatus are currently widespread. In electrophotography, an image comprising an electrostatic field pattern (also referred to as an electrostatic latent image), usually of nonuniform strength, is formed on an insulative surface of an electrophotographic element. The insulative surface typically comprises a photoconductive layer and an electrically conductive substrate. The electrostatic latent image is then developed or visualized into an image by contacting the latent image with a toner composition. Generally, the toner composition contains a resin and a colorant, such as a pigment. The toner image is then transferred onto a transfer medium such as paper and fixed thereon by heating and/or pressure. The last step involves cleaning residual toner from the electrophotographic element.
In general, conventional dry toner compositions are prepared by combining a polymeric resin and a colorant followed by mechanical grinding (particle size attrition). The grinding process typically results in uncontrolled breakage of the particles, yielding toner compositions having irregular shapes with relatively wide particle size distributions.
There is a growing need in the industry for toner compositions that can produce images having improved print quality using lower amounts of dry toner per page. In order to meet these needs, efforts have been made to improve the dispersibility of the colorant in the resin and reducing the overall particle size of the toner composition. However, the current mechanical grinding processes are not able to efficiently produce small particle size toners since the energy consumed in grinding typically increases exponentially with the particle size. Also, the irregularly shaped conventional toner particles cannot pack as well as regularly shaped particles, resulting in higher waste of toner per page.
For this reason, various processes have been developed which produce toner particles having small and/or regular shapes. These processes involve the formation of resin particles in the presence of the colorant. Toners produced using such “in situ” processes are often referred to as “chemically prepared toners” or CPTs. For example, a process has been developed in which a polymer latex is combined with an aqueous pigment dispersion and agglomerated using a coagulant to form polymer particles. Another process involves the aqueous suspension polymerization of a dispersion of pigment in at least one monomer. Also, a pigment/polyester resin dispersion has been prepared and combined with water, followed by evaporation of the solvent. Each of these processes result in small particle size toner compositions having regular shapes. However, for each of these processes, since smaller particles result, the dispersibility of the colorant in the polymer becomes very important in order to maintain or improve the properties of the toner. To provide good dispersibility, high levels of dispersants must be included in the chemical toner processes. This has a negative impact on the overall performance of the toner composition, particularly the viscosity of the mixtures used to prepare the toners as well as the moisture sensitivity of the resulting chemical toner. Other issues have also been found.
Modified pigments having attached organic groups have been disclosed for use in toner compositions. For example, U.S. Pat. No. 6,218,067 discloses, in part, a toner composition comprising the product of a mixture of resin particles and chargeable modified pigment particles. The modified pigment particles comprise at least one organic ionic group attached to the pigment particles and at least one amphiphilic counterion. Also, U.S. Pat. Nos. 5,955,232 and 6,054,238 disclose, in part, toner compositions comprising resin particles and modified pigment particles having attached at least one positively chargeable organic group. In addition, U.S. Patent Publication No. 2002-0011185 discloses, in part, a modified pigment product comprising a pigment having attached at least one organic group represented by the formula -X-Sp-Alk, wherein X, which is directly attached to the pigment, represents an arylene, heteroarylene, or alkylene group, Sp represents a spacer group, and Alk represents an alkenyl or alkyl group containing 50-200 carbon atoms. Toner compositions are also disclosed. Furthermore, U.S. Pat. Nos. 6,337,358 and 6,372,820 and U.S. Patent Publication No. 2002-0055554 disclose, in part, toner compositions comprising modified particles having attached polymeric groups.
While the materials disclosed in these patents and publications provide toner compositions having good overall performance, there remains a need for toners, in particular chemical toners, with properties capable of meeting the increasingly demanding print performance, efficiency, and cost requirements of the industry.